TRANSMISSION LINE AND NETWORKS (EC335)
UNIT I
TRANSMISSION LINE THEORY AND PARAMETERS
PART A
1.What do you mean by a transmission line?
Transmission lines are considered to be impedance-matching circuits designed to deliver power from the transmitter to the antenna and maximum signal from antenna to the receiver.
2.Define characteristic impedance of a transmission line
characteristic impedance is an important electrical property of a symmetrical network and it is defined as the input impedance measured at the input terminals when the output terminal is terminated in the same impedance.
Z0=zy for symmetrical T network
Z=R+jL and Y=G+jC
3.Define propagation constant.
Propagation constant () is defined as the natural logarithm of the ratio of sending end current to the receiving end current.
=ln(IS/IR)
=+j
where,
attenuation constant in neper
phase constant in radian
4.Give the equations for voltage and current at any point on the transmission line at a distance S from receiving end.
E=ER coshzys+IRZ0sinhzys
Where,Z0characteristic impedance
Sdistance from the receiving end
ERIR are the receiving end voltage and current respectively
5.Define phase velocity
Phase velocity or velocity of propagation(v) is defined as the velocity which the wave is transmitted in the transmission line. It is given by,
V=
6.Define loading: What are the two types of loading?
The transmission line produces distortion in the signal, which is to be transmitted, and it is undesirable. The condition for distortion less line is RC=LG, ie, R/G>L/C.In order to achieve this condition, extra inductance is added with the line which is called loading. There are two types of loading namely, 1.continuous loading 2.lumped loading
7.What is continuous loading?
Continuous loading is the process of adding inductance by distributing it uniformly along the line. This is accomplished in submarine cables by winding it with a high permeability steel tape such as permalloy.
8.What is lumped loading?
Lumped loading is the process of adding inductance in the line at uniform intervals. This lumped loading is used in open wires / load lines or cables to improve the transmission. The core of the coil is toroidal and made by perm alloy.
9.What are the advantages of continuous loading?
i)The attenuation is independent of frequency and it is same for all frequencies
ii) The value of can be reduced by increasing L provided R is not increased greatly
iii) The increase in the inductance up to 100mH per unit length of the line is possible.
10.What are the disadvantages of continuous loading?
i) This method is very expensive. The existing lines cannot be modified by this
method
ii) Extreme precision care should be taken while manufacturing continuous loaded
cable. Otherwise it becomes irregular
iii)for an ac signal there will be large eddy current and hysterisis loss. Eddy current loss varies direcly with square of frequency while the hysterisis loss varies directly with frequency.
11.Write the Campbell’s equation
coshN =ZL/(2Z0)sinh(N)+cosh(N)
where,
Nnumber of miles between the loading coils
Propagation constant for the loaded line
ZLInductance of the loaded coil
Z0characteristic impedance
12.What are the advantages of lumped loading?
i)There is no practical limit to the value by which the inductance can be increased
ii)The cost involved is small
iii)With this method, the existing lines can be tackled and modified
iv)Hysteresis and eddy current losses are small
13.What are the disadvantages of lumped loading?
i)After particular frequency the line acts as a low pass filter and above the cutoff frequency the attenuation increases
ii)The cutoff frequency must be at the top of voice frequency. Hence fractional loading must be used. Care must be taken while installing the lumped inductors so as to maintain the exact balancing of the circuit
14.What do you mean by smooth line?
Finite line terminated in Z0 without having any reflection is called smooth line. The waves travel smoothly along the line and energy is absorbed in the load Z0 without any reflection
15.Define reflection coefficient
The ratio of amplitudes of the reflected and incident voltage waves at the receiving end of the line is frequently called the reflection coefficient
K=reflected voltage at load / incident voltage at load
K=(ZR-Z0)/(ZR+Z0)
16.Define reflection loss
The reflection loss is defined as the number of nepers or decibels by which the current in the load under image matched conditions would exceed the current actually flowing in the load
Reflection loss,nepers=ln(Z1+Z2)/(2Z1Z2)
17.Define reflection factor
It is the ratio of current actually flowing in the load to that which might flow under image matched conditions
K=I2/I2 =2Z1Z2/(Z1+Z2)
18.Define insertion loss
The insertion loss of a line or network is defined as the number of nepers or decibels by which the current in the load is changed by insertion.
Insertion loss in nepers=ln(1/KS) + ln(1/KR) –ln(1/KSR) +l
Where, KS,KR & KSR are reflection factors
19.Define return loss
Return loss indicates the ratio of the power at the receiving end due to incident wave to the power reflected by the load
Return loss=10 log(P1/P3)=20 log(1/k)
kreflection coefficient
20.Give the equation for input impedance interms of Z0 & k
ZS=Z0 [(e l +ke-l)/(el-ke-l)]
21.What are the different types of transmission lines used in practice?
1. open wire line
2. cables
3. co-axial cables
4. wave guides
22.Give the relation between neper and dB and prove the relation
1 neper=8.686 dB
let m be represented in neper and dB,
neper=ln m------------(1)
dB=20 log m----------(2)
ln m=2.303* log m
log m=(1/2.303) ln m
20 log m=(20/2.3030) ln m
dB=8.686 neper
23.State the T and section equivalent parameters of a ine of length l terminated in Z0
T section
Z1/2=Z0 tanh(l/2)
Z2=Z0/sinh(l)
section
Z1=Z0 sinh(l)
Z2=Z0 coth(l/2)
24.Define group velocity
The velocity which is produced by a group of frequency traveling along the system is called group velocity. It is defined as,
Vg=d/d
25.State the disadvantages of reflection?
1. reflected wave appears as echo at the sending end
2. the efficiency is reduced
3. the output reduces as load rejects part of the energy
4. if generator impedance is not Z0, then reflected wave is again reflected at sending end as new incident wave. This continuous back and forth till all the energy is dissipated as the line losses
PART B
1. Derive the relations to find the voltage and current at any point on the transmission line at a distance S from the receiving end.
2. Derive an equation for for a continuous loaded cable.Derive the Campbell’s equation for lumped loading
3. Derive an expression for reflection loss, reflection factor and insertion loss
4. Derive an expression for input impedance interms of z0 and k
5.Derive the expressions for for a distortion less line
UNIT:II
THE LINE AT RADIO AND POWER FREQUENCIES
Part A
1Define skin effect?
At very high frequency, skin effect is considerable. Skin effect is defined as the effect in which the current may flow on the surface of conductor . Now the internal inductance of conductor becomes zero.
2.What are the standard assumptions made for the analysis of the performance of transmission line?
1) At very high frequency, skin effect is considered. Hence the internal inductance become zero.
2) Due to skin effect, resistance R increases with f^(1/2). But the line reactance wL increases directly with frequency f ie, L>>R
3) The line at radio frequency is constructed such that the leakage conductance G may be considered zero.
موضوع: TRANSMISSION LINE AND NETWORKS (EC335) 
الخميس 29 نوفمبر - 18:45
